PostCSS doesn't run against @import url() local css.

[MadMaxMcKinney]

Describe the bug
In my +layout.svelte root file I have this import statement for my global css

<style lang="postcss">
    @import url('../app.css');
</style>

However the imported css is not processed via PostCSS. If I directly in-line the css instead of importing it, PostCSS runs correctly.

Specifically concerned about nesting, here's my svelte.config

import adapter from '@sveltejs/adapter-auto'
import preprocess from 'svelte-preprocess';
import nesting from 'postcss-nesting'

/** @type {import('@sveltejs/kit').Config} */
const config = {
	// Consult https://kit.svelte.dev/docs/integrations#preprocessors
	// for more information about preprocessors
	preprocess: preprocess({
        postcss: {
            plugins: [nesting()]
        }
    }),
    kit: {
		adapter: adapter()
	}
};

Expected behavior
I'd like to keep my general css in it's own file, what is the best way to work here? I'd expect the imported css to be processed.

[pooledge]

Experiencing the same, but with a global import within script:

<script lang="ts">
	import '$styles/layout/index.scss';
</script>

I'm postcss'ing with mqpacker and sorting: true in order to get column widths working via placeholders, and the impact of this bug is quite big. Just not sure anymore where it started.